There will be increasing opportunities for specialists in areas like diagnostics, aircon and specific makes like Land Rover, BMW etc.
However, there will always be a call for the ordinary garage as well. Someone who owns a Mercedes may well look for a specialist in that marque but a Ford driver won't. He just drives a car, he has bought it to go from A to B and doesn't want the fuss of using a specialist for it and then finding another one for his wifes Corsa! When his car goes wrong he doesn't want to find a diagnostic specialist and he doesn't want to find an aircon specialist when that fails. Your average joe public car driver wants a one stop shop and the fact that this isn't as easy as it used to be is a problem for the trade - not the punter.
The answer? It's probably a combination of things. All garages will do a certain amount of diagnostics, condemming a wheel bearing is a diagnostic procedure and according to their skill and equipment level they will diagnose what they can. When they cannot do it themselves they will either recommend or more often sub-contract to a specialist who does have the extra skills and equipment to complete the diagnosis or repair that is needed.
You cannot expect a general workshop to give all diagnostic work to a specialist any more than you could expect me to turn down a well paid general service for one of my customers who I first saw for a diagnosis!